Headends are coupled to multiple set-top boxes via in-band and out-of-band channels. Forward out-of-band channels convey (downstream) signals from headends to digital home communication terminals (DHCTs) while backward out-of-band channels convey (upstream) signals from digital home communication terminals to headends. Downstream signals usually include media signals, data signals, system messages and the like.
In-band channels are usually a part of communication networks such as hybrid fiber coax (HFC) networks. In-band channels are utilized for the transmission of digital broadcast programs and/or analog broadcast programs. The bandwidth of the out-of-band channels, and especially the reverse out-of-band channels is limited. This limited bandwidth is shared by many digital home communication terminals. Media access control (MAC) protocols are required for sharing a single upstream out-of-band channel between multiple digital home communication terminals.
General Instrument has developed an out-of-band format known as SCTE DVS 178. This format uses polling and ALOHA MAC schemes. In polling mode, a digital home communication terminal, such as a set top box, can transmit upstream data only when polled by the headend controller. ALOHAALOHA is also known in the art as Carrier Sense Multiple Access with Collision Detect (CSMA/CD). CSMA/CD was first utilized in Ethernet networks. In order to transmit a packet over a shared communication channel a transmitter needs to sense that no packet is currently being transmitted. This scheme can cause collisions, as more than a single transmitter can sense a lack of activity and try simultaneously to transmit packets. The scheme detects collisions, aborts the colliding transmission and reschedule the transmission for a random time later. It is noted that in Ethernet networks carrier sense is used to detect whether no packet is currently transmitted while in cable systems a MAC controller transmits downstream system messages reflecting the status of the channel.
Scientific Atlanta adopted an out-of-band format known as SCTE DVS 167. This format has three modes: (i) a contention mode that uses slotted ALOHA, (ii) reservation mode in which a transmitter requests a guaranteed transmit opportunity and transmits only if the request defining allowable time slots for transmission is accepted, and (iii) Contentionless mode that guarantees a certain number of slots to a connection.
Slotted-ALOHA differs from ALOHA as the time line for transmission is divided into time slots. A transmission of data can be initialized at the beginning of a time slot, and not at any arbitrary time, as in ALOHA. A time slot allows for a transmission of a packet of a predefined length and also includes a guard period. Packets that are longer than the predefined length are segmented prior to the transmission and are reassembles after being received by a receiver.
The three modes of SCTE DVS 167 format require precise synchronization and scheduling between different transmitters. SCTE DVS 178 MAC does not include a transmission of timing information.
System messages such as conditional access entitlement information, system information, program-guide information, emergency alert system information can be delivered via forward out-of-band channels or by in-band channels.
The transmission and especially the reception and processing of forward out-of-band transmitted system messages is defined (and accordingly limited) by the out-of-band standards utilized in the system. For example, DVS 178 compatible systems are configured to receive and adequately process DVS 178 compatible system messages.
In-band transmission of system messages has some disadvantages. Michael Adams describes some of these disadvantages at “OpenCable Architecture”, Cisco Press, 2000, pages 95-96. For example: each set top box has an in-band channel that can be tuned to a single channel out of multiple available channels. As the status of each set-top-box is not known to the headend, system messages must be simulcast at each possible channel. This simulcast is bandwidth consuming and requires message insertion equipment for every channel. Some of these channels are analog channels which have a very limited capacity to carry data.
A transmission of system messages, either in in-band or out-of-band channels has another further disadvantage. As the headend in not aware whether a system message was received by a digital home communication terminal the system messages are repeatedly transmitted. The system messages are transmitted in a circular queue that includes system messages to each digital home communication terminal that is coupled to the headend. As the amount of digital home communication terminals increases this causes considerable queuing delay.
There is a need to provide a system for enhancing the capabilities of DVS 178 compatible digital home communication terminals.
There is a need to provide timing information to digital home communication terminals in an efficient manner.